1. Field of the Invention
The present invention is directed to fiber optic sensors, and in particular, to fiber optic microphones for measuring fluctuating pressures.
2. Description of the Related Art
Fiber optic pressure sensors are known and fall primarily into either of two categories: interferometric sensors or intensity-modulated sensors (fiber optic lever). Interferometric sensors respond to a change of phase of light in a sensing fiber relative to that in a reference fiber. The fiber optic lever, on the other hand, responds to changes in intensity of the light reflected from a vibrating surface.
In the fiber optic lever, light from a light source is directed through a transmitting fiber, reflected from a mirror on a membrane, intercepted by receiving fiber(s), and detected by a light receptor. When the membrane vibrates, it modulates the intensity of the received light. A transfer coefficient of light, which is the ratio of total radiated light power from the proximal tip at the photodiode of the receiving fibers to the total incident power into the transmitting fiber, may be determined. The transfer coefficient is lightly sensitive to the displacement between the mirror and the distal end of the fibers.
Since prior art fiber optic levers use either a massive optical reflector or an unstretched membrane as the pressure sensing element, they suffer limited frequency response. Further, those using an unstretched membrane have no means of damping the membrane motion to suppress sharp peaks in the frequency response. Still further, these sensors lack means for on-line calibration.
Interferometric fiber optic sensors enjoy the advantage of higher sensitivity than known fiber optic levers, but in field applications suffer the serious disadvantage that they respond to vibration and thermal gradients along the entire length of the fibers. The fiber optic lever, on the other hand, is sensitive only to events taking place in the small gap between the distal end of the fibers and the mirror on the vibrating membrane.
Other sensors are known which are based upon a constitutive property such as piezoelectricity, piezo-resistance, or magnetostriction. For example, non-fiber optic pressure sensors include the piezoresistive transducer manufactured by Kulite Semiconductor Products, Inc. or Endevco Corp.; the piezoelectric transducer manufactured by PCB Piezotronics, Inc.; the ceramic microphone manufactured by General Radio Co.; and the condenser microphone manufactured by Bruel & Kjaer, Inc. These sensors are inherently low-temperature devices, although some commercially available sensors permit operation at moderately high temperatures. For example, the Kulite HEM-375 Series or PCB Piezotronics Model 123A can operate at 500.degree. F. The attachment of a water-cooled jacket to such a sensor will permit operation at higher temperatures. However, the attachment of a water-cooled jacket to piezoelectric, piezoresistive, or magnetostrictive sensors has two main disadvantages: first, the water-cooled sensor creates a cold spot in the high-temperature test environment and may profoundly influence the fluctuating pressures to be measured; secondly, the volume occupied by the cooling jacket prevents close sensor proximity which is required in some applications.
The condenser microphone is provided with a backplate and a membrane which create a capacitor wherein pressure changes are measured by measuring capacitance changes. Although the condenser microphone exhibits good performance in regard to bandwidth, it suffers a serious limitation. Since capacitive loading by the connecting cable reduces the microphone sensitivity and contributes to 60 Hz interference, it is necessary that a signal conditioning electronic circuit, also known as a preamplifier, be located in close proximity to the pressure sensing element. This makes the condenser microphone impractical for use in confined spares. Further, the input capacitance of the preamplifier limits the minimum practical size.
U.S. Pat. No. 4,599,711 to Cuomo discloses a multi-lever miniature fiber optic transducer. A bifurcated fiber optic transducer comprises one transmit fiber and at least one pair of receive fibers, each receive fiber pair having two fibers with different core diameters. The transmit and receive fibers are separated at one end and combined at the distal end in the vicinity of a miniature reflective surface sensitive to axial motion caused by minute pressure changes, either in air or water, such that any displacement of the reflector from equilibrium will increase or decrease different illuminated areas of the receive fibers which can be used to generate a processed output signal proportional to this motion, thus providing a sensitivity and an output independent of variations at the input. The reflector is provided in an unstretched state at the distal end of the fiber through an optically clear bonding compound.
Kaman Sciences Corporation markets an eddy-current sensor capable of operating at temperatures up to 2000.degree. F. as the "KP-1911 Series Basic System Configuration." A disadvantage of the Kaman sensor is the limited frequency response. Eddy currents are generated in a metallic button attached to a vibrating membrane, and thus the button mass-loads the membrane and severely limits the response at high frequencies.
U.S. Pat. No. 4,149,423 to Zuckerwar discloses a high-temperature microphone system. The sensor is a condenser microphone driven by a radio frequency oscillator through a bulky half-wavelength transmission line. Consequently, the sensor cannot attain a small size, high frequency response, or high operating temperature desired in many applications.
U.S. Pat. No. 4,687,927 to Iwamoto et al discloses a pressure measuring system in which light is transmitted through an optical fiber to an unstretched diaphragm having a reflective surface. The diaphragm reflects light to another optical fiber and to a photosensitive element. A second reference optical fiber is provided to compensate for system fluctuations. The reflective surface area is large compared with the area of the optical fibers.
U.S. Pat. No. 4,926,696 to Haritonidis et al discloses a fiber optic micropressure transducer. A thin diaphragm is positioned across a chamber from an optical surface and micromachined into a substrate. A fiber optic assembly is mounted facing one side of the diaphragm. Coherent light from a fiber is partly reflected by the diaphragm and recombines in the fiber to produce an interference pattern which is indicative of diaphragm deflection.
U.S. Pat. No. 4,932,263 to Wlodarczyk discloses a temperature compensated fiber optic sensor comprising a wall enclosing a chamber formed from two members micromachined in silicon or a similar substance, a membrane which may have an optical grating formed thereon, and a fiber optic fiber which extends into the cavity parallel to the membrane. Light is injected into the fiber with a wavelength that couples with the grating on the membrane. The coupling varies with pressure outside the chamber.
U.S. Pat. No. 4,473,747 to Brogardh et al discloses a semiconductor structure for a fiber optic pressure sensing element. The sensor comprises an unstretched diaphragm consisting of at least two layers applied to an apertured substrate, at least one of the layers having luminescent properties. Stresses in the diaphragm change the spectrum of luminescent light emitted by the active luminescent layer. The luminescence may be sensed by an optical fiber. The sensitivity of the diaphragm is chosen by the cross-sectional area of a cavity formed in the sensor and the thickness of the diaphragm.
In pressure sensors, it is desirable to calibrate the sensor to obtain a true reading of the pressure fluctuations. Calibration has been performed by attaching an electrode to the outside surface of the pressure-sensing membrane. This suffers a disadvantage at high frequencies because the electrode interferes with acoustical signals. Further, since the electrode is not a part of the sensor, the measurements are affected.
Fiber optic sensors which measure fluctuating pressures by detecting changes in light transmission from a membrane require a membrane having good reflectivity. Typically, the membrane must be processed to have the requisite reflectivity. For example, polishing may be required to produce a highly reflective surface.
A number of methods are known for polishing surfaces. For example, U.S. Pat. No. 4,098,031 to Hartman et al discloses a method for lapping and polishing a semiconductor material. A wafer is bonded to a spacer which is capable of accommodating surface irregularities by applying a suitable adhesive such as wax. The spacer and the wafer are then mounted to a mounting plate which is inserted into a lapping fixture for lapping and polishing.
U.S. Pat. No. 4,258,508 to Wilson et al discloses a method of holding down wafers mounted on a mounting plate without the use of wax. Once mounted, the wafers are mounted on a polishing machine, and thereafter, the wafers are cleaned with hydrofluoric acid.
U.S. Pat. No. 4,512,113 to Budinger discloses a workpiece holder for a polishing operation provided with a template releasably secured to a carrier by an adhesive layer. The template has a plurality of equally spaced holes defining a cavity. Inserts are provided in each cavity and include a vacuum material bonded to a fixturing material by an adhesive layer. Four pieces are releasably secured to the fixturing materials and extend beyond the exposed major face of the template. After mounting the workpieces, they can be polished in a polisher.
There is a need for sensors capable of detecting fluctuations in pressure. In particular, there is a need for sensors which are capable of achieving a high frequency response, are small in size, and are capable of operating at high temperatures. Further there is a need for techniques for processing membranes used in such sensors so that their surfaces have the requisite reflectivity for detecting changes in pressure. Still further, it is desirable to provide means for calibrating such sensors.